Homeostasis requires the immunologically silent clearance of apoptotic cells (AC) before they become pro-inflammatory necrotic cells. CD300f is a phosphatidylserine receptor known to positively regulate efferocytosis by macrophages, and CD300f gene-deficient mice are predisposed to develop a lupus-like disease. We showed that CD300f is expressed on both macrophages and dendritic cells (DC), and differentially regulates their efferocytosis. CD300f functions as an activating receptor to enhance macrophage efferocytosis, but inhibits the AC engulfment by DC. Although it is not clear at this point how CD300f differentially regulates efferocytosis in macrophages and DC, of note is the fact that CD300f has five tyrosine-based motifs in its cytoplasmic tail: two immunoreceptor tyrosine-based motifs (ITIM), an immunoreceptor tyrosine-based switch motif (ITSM), a PI3K-binding motif and a growth factor receptor-bound protein 2 (Grb2)-binding motif. AC binding to CD300f leads to phosphorylation of the PI3K-binding motif thereby promoting PI3K recruitment and activation, which initiates the signaling events required for AC engulfment; each of the other four tyrosine-based motifs initiate inhibitory signals that serve to suppress efferocytosis. Whether AC are engulfed or not depends on the available tyrosine kinases and/or downstream signaling capacity within the phagocytic cell. In macrophages the balance of these capabilities favors AC engulfment, while the opposite is true for DC. Our data show that CD300f-deficient macrophages are less efficient in clearing AC and that the engulfment of AC by DC is enhanced. Efferocytosis by DC is highly relevant to T-cell priming, since DC are efficient at processing and presentation of engulfed antigens. CD8+ DC engulf AC and migrate to the T-cell zone of the spleen for cross-presentation of AC-associated antigens. In correlation with the increased phagocytic capability of CD300f-deficient DC, more DC engulfed AC in the spleen of CD300f-deficient mice, compared with WT mice. The increased availability of AC, combined with enhanced efferocytic potential of CD300f-deficient DC, likely leads to elevated presentation and cross-presentation of AC-derived self-antigens to T cells, resulting in the observed sustained presence of activated T-cell populations and an expanded population of memory T cells in aged mice. This creates more potential to trigger self-reactive B cells to secrete auto-antibodies, which would explain the increased levels of ANA that we observe in aged mice. The increase in the memory T-cell compartment and auto-antibody levels did not lead to overt signs of autoimmune disease development in CD300f-deficient mice. In elderly people, it has also been observed that there are higher levels of auto-antibodies without clinical relevance. Such individuals also have higher levels of memory and effector T cells, yet are in general hypo-responsive and referred to as immunosenescent. Other than the higher frequency of memory and activated T cells in aged CD300f-deficient mice and the presence of elevated ANA, the T-cell proliferation in response to TCR stimulation, the activation status of macrophages or DC based on co-stimulatory molecule expression, and the levels of inflammatory or anti-inflammatory cytokines were similar between CD300f-deficient and WT mice, suggesting an immunosenescent status in aged mice. Thus, our data indicate that the autoimmune-risk status generated by CD300f deficiency occurs in the context of immunosenescence, which explains the absence of overt disease pathologies. Although CD300f-deficient mice do not develop overt disease, they clearly are more prone than WT mice to develop overt autoimmune symptoms if stressed. CD300f-deficient mice developed a more severe autoimmune inflammation than the WT mice, when the mice were stressed with excess AC in the pristane-induced autoimmune disease model, which fits the factthat repeated exposure to AC leads to exacerbated disease in autoimmune-prone mice. Moreover, our previous study showed that combined CD300f/FcgRIIB deficient mice are predisposed to develop more severe splenomegaly and have a higher fatality rate than in Fcgr2b deficient mice. We performed an in-depth analysis to identify cellular and molecular abnormalities behind this predisposition. In the double-deficient mice, germinal center B cells and plasma cells were expanded and inflammatory cytokines were increased. An enhanced accumulation of AC was found in spleen tissues of double-deficient mice. These data indicate that the autoimmune-risk status due to CD300f deficiency advances to a fully activated autoimmune response once the immune activation threshold is decreased by combining CD300f deficiency with FcgRIIB deficiency, or the immune system is over-loaded with excess AC. Receptor CD300b is implicated in regulating the immune response to bacterial infection by an unknown mechanism. We identified CD300b as a lipopolysaccharide (LPS)-binding receptor and determined the mechanism underlying CD300b augmentation of septic shock. In vivo depletion and adoptive transfer studies identified CD300b-expressing macrophages as the key cell type augmenting sepsis. We showed that CD300b, and its adaptor DAP12, associated with Toll-like receptor 4 (TLR4) upon LPS binding, thereby enhancing TLR4-adaptor MyD88- and TRIF-dependent signaling that resulted in an elevated pro-inflammatory cytokine storm. LPS engagement of the CD300b-TLR4 complex led to the recruitment and activation of spleen tyrosine kinase (Syk) and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K). This resulted in an inhibition of the ERK1/2 protein kinase- and NFkappaB transcription factor-mediated signaling pathways, which subsequently led to a reduced interleukin-10 (IL-10) production. Collectively, our data describe a mechanism of TLR4 signaling regulated by CD300b in myeloid cells in response to LPS.